Problem with WRT54 GS and Satori 4.0

Hello there.First I want to say sorry for my bad englisch but I hope you will understand me.I use a WRT54GS Router with Satori 4.0.One of the Antennas is an original and the other is a Yagi Antenna to connect a friend 50 meters far away(My friend has a Yagi Antenna too both Antennas have +18db ).the conection quality is always very good 80%,but every 2 or three minutes the router disconnects but only the computer of my friend.My Laptop still works great.And if he wants to play online the connection stopps immidiatly.Could this be the Hidden Station Problem I read about.Please Help me to solve this problem